Clamp-around DC ammeters are now available at reasonable prices, under $150. These are a big win when working with medium and high current DC circuits. They're useful down to about 1A. Note that most clamp-around meters are AC only, but DC ones are available. You usually get all the usual DVM functions built in, too.
If you want to build one yourself, look here:
What`s wrong with shunting-off a small bit of the current?
----------------------------------------------------------------------- Ashley Clarke
Even cheaper is a 4' length of 14ga house wire (THHN). This is 10 milliohms (0.01 ohms), within a couple of percent. If you wrap the wire in a coil, you'll get a little bit of added inductance to smooth out your PWM control at the same time. Put it in the ground return from the H-bridge, and you'll get a nice 0.2 volt signal referenced to ground at 20 amps, which is big enough to measure easily without unduly affecting the performance of the motors.
-- Mark Moulding "I prefer heaven for climate, hell for company."
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